I think you may mean the RFC/RNAS, a light blue-grey uniform was introduced shortly after the formation of the RAF, so after 1st April 1918 much later this was changed to the darker RAF Blue Grey...
The earliest RAF uniforms were green as per the Army/RFC uniform - however to depict the RAF from 1918-1920, a strong mix of RFC and RNAS uniforms would still very much be evident, photos of early RAF show a complete mishmash of uniforms with RFC and RNAS uniforms mixed with bits of RAF, or even Army, a lot of RFC and RNAS men were still very loyal and kept their previous uniforms as much as possible - the RFC maternity tunic became known as the 'eternity' tunic, and in 55 Squadron, a DH4 day bomber Squadron, the first man to turn up in the blue RAF uniform was dumped in a muddy ditch!
This book is very handy for early RAF uniforms. If you live near Stafford in the UK, that is where the RAFM's uniform collection is, the staff there are very friendly and helpful
Excerpt from a song by 55 Squadron regarding the new RAF uniforms;
"Look at those awful fellows, clad in pale blue, Aren't they awful dressy, don't you like the hue? Surely they should not be wandering around the land, Memberes of some uninterned Hungarian band. They are not musicians, nor commissionaires - You mustn't judge a person by the things he wears. I asked one for a taxi, and he feigned that he was deaf, So I guess he is a member of the R.A.F."
The name of the song is 'Oh Cecil!', which may be a reference to Hotel Cecil where the headquarters of the RAF was